Control amplifier



E. W. KELLOGG CONTROL AMPLIFIER Filed 001;. 14, 1941 Sept. 5, 1944.

V 3rwentor l'dwm diffislla 1 I] ttom eg Patented Sept. 5, 1944 UNITED STATE PATENT orr cs CONTROL ADIPLIFIER Edward W. Kellogg, Indianapolis, Ind., assignor to Radio Corporation of America, a corporation of Delaware Application October 14, 1941, Serial No. 414,914

8 Claims.

low amplitudes and has a high amplification,

while the other channel is operative at higher amplitudes and has a low amplification. The advantage of such an arrangement is that, when a sound begins or when a very low sound suddenly increases, the ground noise reduction shutters or equivalent apparatus is moved rapidly, thereby avoiding the clipping of peaks on short sounds or on rapidly increasing sounds; while, at higher amplitudes, the ground noise track is kept Just sufliciently clear .of the sound wave peaks without the excessive clearance and consequent ground noise which would result if the initial amplification were used over the entire recording range.

The present apparatus, like the Kreuzer apparatus, uses two channels but differs therefrom in that the high amplification channel is definitely limited in its range of operation and is provided with a much more rapid timing circuit than the other channel.

One object of the invention is to provide .a control amplifier;

Another object of the invention is to provide a ground noise reduction amplifier which will respond more promptly toimpulses of low amplitude;

Another object of the invention is to provide a control amplifier having rapid timing at low amplitudes and slower timing at high amplitudes.

Other and incidentalobjects of the invention will be apparent to those skilled in the art by a reading of the following specification and an inspection of the accompanying drawing in,

which Figure 1 is a characteristic curve of the amplifier showing the relation between signal voltshutter, or the operation of similarly controlled apparatus;

Figure 2 is a schematic wiring diagram of one form of my improved amplifier;

Figure 3 shows a modified form thereof, differing slightly from Figure 2; and

Figure 4 shows a form corresponding to Figure 2 but using a full wave rectifier.

In Figures 2 and 3 similar parts are indicated by the same reference numerals.

In the form of the invention shown in Figure 2, the audio frequency input to the amplifier,

which is usually the output of a voltage amplifier or of the recording amplifier, is fed to the apparatus through the transformer [0, which is provided with, two secondary windings. The

output from the transformer Ill is applied to the rectifier II, which is of the type normally used as a full wave rectifier. It will be noted that, in the present instance, the two portions. of therectifier are used to rectify the same half wave instead of opposite half waves, as is more customary. The output from the lower secondary of the transformer I0 isfed through the lower portion of the circuit, which is constructed inthe manner now customary for ground noise reduction amplifiers. The output of the rectifier is filtered by the capacitors l3 and. I5 and the resistor M, the capacitors being shunted by the 'resistor l2. The output from this filter circuit is used to control the amplifier tube IS, the

output of which is used to control the ground noise reduction apparatus or other device to be controlled and indicated at H. In this filter circuit, the charging time of the capacitor I3 is determined by the internal resistance ofv the rectifier I I, the resistance of the secondary of the transformer l0, and the value of the resistor l2; while the charging time of the capacitor I5 is determined bythese factors and also the value of the resistor I4. The discharge of the capacitor I5, which merely controls the grid potential in the tube I6, is determined by the rate of discharge of the capacitor l3 through the resistor l2 and the discharge of the capacitor I5 through the resistors l4 and I2 in series. The timing of this portion of the circuit may be substantially circuits ordinarily used in the art. Y

The output of the upper secondary of the transformer I0 is used to control the output of.the tube 25 and is passed through an entirely ,different timing circuit. The output from the upper secondary of transformer l0 may be at a considerably higher voltage than that of the lowor secondary, or the output voltage of the rectifier I i from this portion of the transformer secondary may be amplified by the tube i9. This portion of the circuit provides the high speed and high gain initial opening shown in the portion A of the curve ofFigure 1. Since this portion of the circuit works only over the lower portion of the sound recording amplitude, it is not necessary that it should have as effective filtering as the .lower portion of the circuit, and the timing of this portion of the circuit may, accordingly, be speeded up byreducing the filtering. The output from the upper secondary of the transformer "I,

'after being rectified, is, accordingly, filtered only by the capacitor I 8 shunted by the resistor 29,

and these elements may be of such dimensions as to give a much more rapid response than the filter circuit i2, i3, l4, IS. The portion of the output of the rectifier filtered by 29 and i8 is applied to the grid of the. tube l9, which is an ordinary triode, as shown. This tube has a relatively low plate voltage, for exampleoi' the order of 20 volts, applied to it through the resistor 2!, the grid bias voltage being determined by the resistor 20. Instead of the output of this tube being taken from the plate, as usual, use is made of the drop across the resistor 20,.which is a relatively large portion of the applied plate voltage. 22 and the battery 24 to the grid of the tube 25. Any other. appropriate bias means may be substituted for the battery 24, as, for example, a battery shunted by a potentiometer, or one of the customary sources of rectified and filtered alternating current. The filtering for the initial opening of the device I! is determined by the values of the resistor 22 and the capacitor 23 and is made considerably faster than the timing of the lower filter circuit, first described. Since the maximum possible jump in voltage across the resistor 20 is limited to a predetermined magnitude by the plate voltage applied to the tube IS, the maximum rate of change of voltage across the capacitor 23 is limited and, if the values are properly chosen, this will ensure that the initial opening circuitcannot cause shutter thump no matter how much gain is used in the upper secondary of the transformer I or in the tube l9. Tests have shown that the audibility of the opening of the shutter or other ground noise reduction apparatus is determined by the rate of change of shutter current. It may be desirable to give the filter circuit 29', la a faster rate of discharge than the circuit 12, I3.

of opening and closing times to be secured in the main timing circuit than can be secured by the arrangement shown in Figure 2.

It will be apparent that either or both of the rectifiers which charge the condensers i3 and I8 may be of the full wave type instead of half wave rectifiers as shown in Figs. 2 and 3. In

general, it is desirable that the quick acting,

limited amplitude circuit comprising elements [8, i9, 29 in the figures, be charged through a full wave rectifier 28, since this results in producing the small initial opening as soon as a wave of either polarity arrives at the input of the system,

9 whereas with a half-wave rectifier, nothing hap- This voltage is applied through the resistor v pens until a wave of certain polarity arrives. Such an arrangement is illustrated in Fig. 4 which is in other respects identical with Fig. 2.

I claim as my invention:

1. An amplifier of the class described having independent parallel electrical channels between the input and output thereof, one of said channels being effective at low amplitudes only and having relatively fast timing, and the other channel being effective at higher amplitudes and having slower timing.

2. An amplifier of the class described having independent parallel electrical channels between the input and output thereof, one of said channels being effective at low amplitudes only and having relatively fast timing and filtering circuits, and the other channel being efiective at higher amplitudes and having slower timing.

3. An amplifier of the class described having independent parallel electrical channels between the input and output thereof, one of said channelsbeing effective at low amplitudes only and having relatively fast timing and filtering, and the other channel being. effective at higher amplitudes and having slower timing and more effective filtering.

4. An amplifier of the class described having independent parallel electrical channels between the input and output thereof, one of said chan- It will be apparent to those skilled in the art that any or all of the various resistors shown in this circuit may be made adjustable, if desired, so that the timing maybe adjusted to the, best value for the particular type of recording being done at any particular time.

The relatively poor filtering in the circuit l1, I8 is no disadvantage at higher amplitudes since, as shown in Figure 1, the tube l9 cuts oil at the top of the portion A of the curve, and the impulses which may not be adequately filtered. in the filter circuit are not transmitted therethrough.

The form of the invention shown in Figure 3 is substantially identical with that shown in Figure 2 with the exception that an inductance 2! is substituted for the resistor ll inthe main tim- I ing circuit, and the resistor, i2 controlling the discharge time of the capacitor II is moved to shunt the capacitor ll instead 01' the capacitor I3. This arrangement wili-permitdiil'erent ra i s nels being effective at low amplitudes and having a relatively sharp output limit at a predeterindependent parallel electrical channels between the input and output thereof, one of said channels being eflfective at low amplitudes only and having a relatively sharp cut-oil at a predetermined amplitude, relatively fast timing and high amplification, and the other channel being effective at higher amplitudes and having slower timing and less amplification.

7. An amplifier of the class described having independent parallel electrical channels between the input and output thereof, one of said chanles having a limited output and high amplification, whereby its output limit is attained with relatively low signal input, and the other channel being capable of higher output, but having lower gain, both of said circuits being provided with filters which cause a time lag between input and output, the filterfor said-limited amplitude circuit being designed for less time lag. than the filter for said lower gain circuit.

8. An amplifier of the class described having independent parallel electrical channels between the input and output thereof, one of said channels being efiective at low amplitudes only-and having a relatively sharp cut-oil at a predetermined amplitude, relatively fast timing and high amplification and including a full wave rectifier,

and the other channel being effective at higher amplitudes and having slower timing and less 5 amplification.

EDWARD W. KELLOGG. 

